This invention relates to a novel fluorinated cation exchange membrane having both carboxylic acid groups and sulfonic acid groups, intermediates and starting materials for production thereof and also to processes for producing such materials. This invention also concerns a novel fluorinated cation exchange membrane having sulfonic acid groups with a high ion-exchange capacity and being provided with physically high strength.
The cation exchange membrane according to the present invention can be used in electrolysis of an aqueous alkali metal halide solution under more severe conditions than those conventionally used while maintaining excellent performance stably for a long time.
In the chlor-alkali industry, wherein caustic soda and chlorine are produced by electrolysis of sodium chloride, the ion-exchange membrane process has recently attracted great attention, because it is more advantageous in various aspects such as prevention of environmental pollution and economical saving of energy than the mercury process and the diaphragm process of the prior art and also because it can produce caustic soda having substantially the same quality as that produced by the mercury process.
The greatest factor which controls the economy of the ion-exchange membrane process is the characteristic of the cation exchange membrane employed. It is necessary for the cation exchange membrane to satisfy the requirements as set forth below.
(1) To have a high current efficiency and a low electric resistance. In order to have a high current efficiency, the membrane is required to have a sufficiently high ion-exchange capacity and low water content, thus giving a high concentration of fixed ions in the membrane. On the other hand, to the effect of lower electric resistance, a higher water content is rather more advantageous. Since the water content will vary depending on the types of ion-exchange groups, the ion-exchange capacity and the concentration of external liquids, it is necessary to select the optimum combination of these factors.
(2) To be resistant to chlorine and alkali at higher temperatures for a long time. A cation exchange membrane comprising a fluorinated polymer can be sufficiently resistant generally under the aforesaid atmosphere, but some membranes may be insufficient in chemical stability depending on the ion-exchange groups contained therein. Accordingly, it is important to select suitable ion-exchange groups.
(3) To be durable for a long time under various stresses working in highly concentrated alkali under the conditions of high temperature and high current density such as a stress of swelling and shrinking, a stress accompanied by vigorous migration of substances to effect peel-off of layers and a stress by vibrations of the membrane accompanied with gas generation to cause bending cracks. Generally speaking, the physical strength of the membrane is different depending on the physical structure of the membrane, the polymeric composition, the ion-exchange capacity and the types of ion-exchange groups. Therefore, it is necessary to realize the optimum selection of these factors.
(4) To be easily produced and low in cost.
In the prior art, there have been proposed several fluorinated cation exchange membranes for use in electrolysis of an aqueous alkali metal halide solution. For example, there is known a fluorinated cation exchange membrane having pendant sulfonic acid groups prepared by hydrolysis of a copolymer comprising tetrafluoroethylene and perfluoro-3,6-dioxa-4-methyl-7-octene sulfonylfluoride.
Such a well-known fluorinated cation exchange membrane containing only sulfonic acid groups, however, is liable to permit permeation of hydroxyl ions migrated and diffused from the cathode compartment therethrough due to the high water content afforded by the sulfonic acid groups. For this reason, such a membrane is disadvantageously low in current efficiency. In particular, when electrolysis is conducted, for example, by recovering a highly concentrated caustic soda solution of 20% or higher, the current efficiency is extremely low to a great economical disadvantage as compared with electrolysis by the mercury process or the diaphragm process of the prior art.
For improvement of such a drawback of low current efficiency, the ion-exchange capacity of sulfonic acid groups may be lowered to, for example, 0.7 milliequivalent or lower per one gram of the H-form dry resin, whereby the water content in the membrane can be decreased to make the fixed ion concentration in the membrane higher than the membrane with higher ion-exchange capacity. As the result, the current efficiency at the time of electrolysis can slightly be prevented from being lowered. For example, when electrolysis of sodium chloride is performed while recovering caustic soda of 20% concentration, the current efficiency can be improved to about 80%. However, improvement of current efficiency by reduction in ion-exchange capacity of the membrane will cause a noticeable increase in the electric resistance of the membrane, whereby no economical electrolysis is possible. Moreover, at any higher value of the electric resistance of the membrane, it is very difficult to prepare a commercially applicable sulfonic acid type fluorinated cation exchange membrane improved in current efficiency to about 90%.
On the other hand, British Pat. No. 1,497,748 and Japanese published unexamined patent application No. 126398/1976 disclose fluorinated cation exchange membranes having carboxylic acid groups as ion-exchange groups. In these membranes, the fixed ion concentration can be made higher due to the lower water content of carboxylic acid groups and therefore the current efficiency can be improved to 90% or higher. Such membranes are also chemically stable under the conditions conventionally used.
When compared at the same level of the ion-exchange capacity, however, the membrane having carboxylic acid groups is higher in electric resistance than the membrane having sulfonic acid groups. Particularly, when used at a high current density, the power unit may be undesirably very high. Moreover, perhaps due to lower water content throughout the membrane, the membrane is prone to shrink when used for a long time in a highly concentrated alkali under severe conditions until it is hardened so as to be brittle, resulting in layer peel-off or crack formation, whereby current efficiency may disadvantageously be lowered.
For improvement of such drawbacks of the membrane having only carboxylic acid groups, there is also known a cation exchange membrane prepared by bonding films of a fluorinated polymer having carboxylic acid groups or groups convertible to carboxylic acid groups (hereinafter referred to as precursors) and a flurorinated polymer having sulfonic acid groups or precursors thereof or by molding a blend of said polymers into a film, followed by hydrolysis, as disclosed by Japanese published unexamined patent applications No. 36589/1977 and No. 132089/1978 and U.S. Pat. No. 4,176,215. However, these polymers are poorly compatible with each other and it is difficult to effect complete bonding or blending. When used under severe conditions, such a membrane is liable to suffer from peel-off or formation of cracks and thereby to cause troubles. The blended product is also entirely insufficient from the standpoint of complete utilization of higher current efficiency of carboxylic acid groups and lower electric resistance of sulfonic acid groups. It merely exhibits the intermediate characteristic to both properties.
The aforesaid Japanese published unexamined patent applications and another Japanese published unexamined patent application No. 23192/1977 also disclose a cation exchange membrane prepared by ternary copolymerization of a vinyl monomer having carboxylic acid groups or precursors thereof, a vinyl monomer having sulfonic acid groups or precursors thereof and a fluorinated olefin, followed by fabrication into a film and hydrolysis. Such a membrane also merely shows the intermediate characteristic.
On the other hand, there are disclosed cation exchange membranes prepared by forming carboxylic acid groups by chemical treatment on one surface of fluorinated cation exchange membranes having sulfonic acid groups, as disclosed by U.S. Pat. No. 4,151,053, Japanese published unexamined patent applications No. 104583/1978, No. 116287/1978 and No. 6887/1979. These membranes, due to the presence of carboxylic acid groups, will effectively inhibit migration and diffusion of hydroxyl ions to exhibit higher current efficiency. Also, since the carboxylic acid groups are present in the thin layer on the cathode side and sulfonic acid groups with higher water content in the residual part of the membrane, the electric resistances of the membrane is low. Thus, these membranes are very excellent from the standpoint of power consumption. However, all of these membranes, while they are stably used with good performance under conventional conditions for a commercially satisfactory term, will suffer under severe conditions of further increased high current density and high temperature from swelling like splotch or formation of water bubbles, peel-off of the carboxylic acid layer from the sulfonic acid layer or formation of cracks in the carboxylic acid layer, thereby causing a decrease in current efficiency, as shown in the Comparative examples.
It has not yet been clarified why such pehnomena are caused. Presumably, the polymeric structure of the fluorinated cation exchange membrane having sulfonic acid groups or derivatives thereof may be one of the factors for such phenomena. That is, these membranes are prepared by chemical treatment of a copolymer of a fluorinated olefin with a sulfur containing fluorinated vinylether as represented by the following formula formed in the shape of a membrane or a hydrolyzed product thereof having sulfonic acid groups: ##STR2## wherein n' is an integer of 0 to 2.
Among said monomers, the monomer of n'=0 will cause the cyclization reaction as shown by the reaction scheme (1) below in the vinylization step as disclosed by Japanese published examined patent application No. 2083/1972. ##STR3## For converting the cyclic sulfone to CF.sub.2 .dbd.CFOCF.sub.2 CF.sub.2 SO.sub.2 F, a number of reaction steps are required to be performed and therefore it is very difficult to produce said monomer in commercial application. Moreover, depending on the conditions, such cyclization will also occur at the time of polymerization and may lower the properties of the resultant polymer.
For this reason, in commercial application, the monomer of n'=1 is conventionally used. With such a monomer, there is the drawback that the ion-exchange capacity of the resultant sulfonic acid type membrane and the membrane having formed carboxylic acid groups by chemical treatment on the surface of the sulfonic acid type membrane can limitedly be increased, as disclosed by the aforesaid Japanese published unexamined patent applications. Furthermore, perhaps due to the presence of the pendant groups: ##STR4## no physically tough membrane can be obtained unless the copolymerization ratio of a fluorinated olefin to the sulfur containing fluorinated vinyl ether is increased to about 6 or more. It is also expected that use of such a monomer may be one of the factors causing peel-off or cracks of the carboxylic acid layer formed when using the membrane having carboxylic acid groups and sulfonic acid groups as mentioned above under more severe conditions than conventionally used. The above drawbacks are further multiplied when the monomer of n'=2 having a larger molecular weight is used.
A copolymer of a fluorinated vinyl monomer having no ether linkage such as trifluorovinyl sulfonyl fluoride with tetrafluoroethylene, as disclosed by U.S. Pat. No. 3,624,053, is deficient in fabricability into a membrane.
Japanese published unexamined patent applications No. 28588/1977, No. 23192/1977 and No. 36589/1977 disclose fluorinated cation exchange membranes prepared from copolymers of fluorinated olefins with fluorinated vinyl compounds represented by the formula: EQU CF.sub.2 .dbd.CX.sup.1 (OCF.sub.2 CFX.sup.2).sub.a O.sub.b (CFX.sup.3).sub.c SO.sub.2 X.sup.4
wherein X.sup.1 is F or CF.sub.3, X.sup.2 and X.sup.3 are F or C.sub.1 -C.sub.10 perfluoroalkyl, X.sup.4 is F, OH, OQ.sup.1, OM and NQ.sup.2 Q.sup.3 (Q.sup.1 is C.sub.1 -C.sub.10 alkyl, Q.sup.2 and Q.sup.3 are H or one of Q.sup.1, and M is an alkali metal or quaternary ammonium), a is an integer of 0 to 3, b an integer of 0 or 1 and c an integer or 0 to 12. However, these prior publications refer to no typical example of a process for preparation of said fluorinated vinyl compounds. Nothing is taught about percursors of said compounds. Moreover, as clearly seen from the description in the specifications of said Japanese published unexamined patent applications, there is only disclosure of the compounds, copolymers and membranes derived therefrom in the Examples and preferred typical examples which are those conventionally known of the formula: ##STR5## wherein a is the same as defined above, namely the group of compounds wherein c is 2, although preferred embodiments are mentioned to be those wherein X.sup.1 =F, X.sup.2 =CF.sub.3, X.sup.3 =F or CF.sub.3, X.sup.4 =F. a=0 to 1, b=1 and c=1 to 3.
In the field of ion-exchange membranes, it is strongly desired to develop a membrane which exhibits high current efficiency and low electric resistance under more severe conditions, has a longer life and can be produced at low cost. The present inventors have made efforts to develop such a membrane and consequently found that the above object can be attained by use of a novel fluorinated vinyl ether compound which is derived from starting materials having specific structure. The present invention has been accomplished based on such a finding.